What your budget is, and what you want to collect.

I have 5 Bally EMs (electro-mechanical) machines, dating from the mid 60's-early 70's. They are reliable, fun to work on, CA-LEGAL, and relatively affordable. Another big advantage is they are pre-computer; in the next few years, the earliest computerized machines will become CA-Legal, and they should be avoided as they are impossible to work on (you'd have to try to find the correct board to replace; nearly impossible...). 25 year old Bally EM's can run anywhere from $200 (not working, in bad shape - fun to fix) to $2,500 for rare, perfect examples. Casino-specific examples can bring more $, but you have to use caution, as glass can be changed & therefore the machine isn't authentic... Also, keep in mind that finding original replacement glass is very difficult (not impossible, but can take years to find the exact glass) - so if the glass is peeling or flaking, talk the price down significantly, or avoid it entirely.

If you can afford more, I'd recommend Mills machines from WWII to the 50's. They are rugged, and can still be found for under $2k. With these machines, you need to decide if you want unrestored, or restored; both can bring a premium to the right buyer. As for me, I'd never want a restored machine; but many people want these old machines to look like they just came out of the factory! Mills machines with unique features; casino glass, and/or unique bonus features are more collectible. Avoid dime machines, as the coins are prone to jamming, as they are so lightweight, and frequently are bent from people using them as screwdrivers... A major thing to look out for, are counterfeit machines, sometimes advertised as "reproduction"; these are completely modern fakes & should never be considered as they are unreliable & won't ever appreciate. Just like with chips, look for a perfect rating, and tons of similar slot sales.

Ebay is a great source for 25y + year machines, but you need to be patient and wait for a good machine that is within driving distance - shipping a slot can add $200-400 to the price. Shipping internationally is prohibitive; upto $1,500 for cargo...